Staffsite STMIK PPKIA Pradnya Paramita DP_14_3
Database Programming with SQL
14-3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
Objectives
This lesson covers the following objectives:
• List four different functions that the ALTER statement can
perform on constraints
• Write ALTER TABLE statements to add, drop, disable, and
enable constraints
• Name a business function that would require a DBA to drop,
enable, and/or disable a constraint or use the CASCADE
syntax
• Query the data dictionary for USER_CONSTRAINTS and
interpret the information returned
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
3
Purpose
• Would it make any difference if a new student ID number was
entered into the school's database when no actual student
enrolled?
• Is it likely that a credit-card company would issue the same
credit-card number to more than one account or that a
business would hire an employee for a department that
didn't exist?
• What do you predict would happen if a business could not
trust the reliability of the information in its database?
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
4
Purpose
• A database system needs to be able to enforce business rules
and, at the same time, prevent adding, modifying, or deleting
data that might result in a violation of the referential integrity
of the database.
• In this section, you will learn how to make changes to table
constraints so that referential integrity and, in turn, database
reliability are maintained when data needs to be changed.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
5
Managing Constraints
• The ALTER TABLE statement is used to make changes to
constraints in existing tables.
• These changes can include adding or dropping constraints,
enabling or disabling constraints, and adding a NOT NULL
constraint to a column.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
6
Managing Constraints
• The guidelines for making changes to constraints are:
– You can add, drop, enable, or disable a constraint, but you
cannot modify its structure.
– You can add a NOT NULL constraint to an existing column by
using the MODIFY clause of the ALTER TABLE statement.
– MODIFY is used because NOT NULL is a column-level change.
– You can define a NOT NULL constraint only if the table is empty
or if the column contains a value for every row.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
7
The ALTER Statement
• The ALTER statement requires:
– name of the table
– name of the constraint
– type of constraint
– name of the column affected by the constraint
• In the code example shown below, using the employees table,
the primary-key constraint could have been added after the
table was originally created.
ALTER TABLE employees
ADD CONSTRAINT emp_id_pk PRIMARY KEY (employee_id);
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
8
Adding Constraints
• To add a constraint to an existing table, use the following SQL
syntax:
ALTER TABLE table_name
ADD [CONSTRAINT constraint_name] type of constraint (column_name);
• If the constraint is a FOREIGN KEY constraint, the REFERENCES
keyword must be included in the statement.
• Syntax:
ALTER TABLE tablename
ADD CONSTRAINT constraint_name FOREIGN KEY(column_name) REFERENCES
tablename(column_name);
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
9
Adding Constraints Example
• Consider the employees database.
• The primary key from the DEPARTMENTS table is entered in
the EMPLOYEES table as a foreign key.
DEPARTMENTS - Parent
DEPARTMENT_ID
90
110
190
DEPT_NAME
Executive
Accounting
Contracting
MANAGER_ID
100
205
-
LOCATION_ID
1700
1700
1700
FIRST_NAME
Steven
Neena
Lex
Shelley
William
LAST_NAME
King
Kochhar
De Haan
Higgins
Gietz
DEPARTMENT_ID
90
90
90
110
110
EMPLOYEE - Child
EMPLOYEE_ID
100
101
102
205
206
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
10
Adding Constraints Example
• The following example demonstrates the syntax to add this
foreign key to the EMPLOYEES table:
ALTER TABLE employees
ADD CONSTRAINT emp_dept_fk FOREIGN KEY (department_id)
REFERENCES departments (department_id)
ON DELETE CASCADE;
DEPARTMENTS - Parent
DEPARTMENT_ID
90
110
190
DEPT_NAME
Executive
Accounting
Contracting
MANAGER_ID
100
205
-
LOCATION_ID
1700
1700
1700
FIRST_NAME
Steven
Neena
Lex
Shelley
William
LAST_NAME
King
Kochhar
De Haan
Higgins
Gietz
DEPARTMENT_ID
90
90
90
110
110
EMPLOYEE - Child
EMPLOYEE_ID
100
101
102
205
206
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
11
Adding Constraints Conditions
• If the constraint is a NOT NULL constraint, the ALTER TABLE
statement uses MODIFY in place of ADD.
• NOT NULL constraints can be added only if the table is empty
or if the column contains a value for every row:
ALTER TABLE table_name
MODIFY (column_name CONSTRAINT constraint_name NOT NULL);
ALTER TABLE employees
MODIFY (email CONSTRAINT emp_email_nn NOT NULL);
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
12
Why Enable and Disable Constraints?
• To enforce the rules defined by integrity constraints, the
constraints should always be enabled.
• In certain situations, however, it is desirable to temporarily
disable the integrity constraints of a table for performance
reasons, such as:
– When loading large amounts of data into a table
– When performing batch operations that make massive
changes to a table (such as changing everyone's employee
number by adding 1,000 to the existing number)
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
13
Dropping Constraints
• To drop a constraint, you need to know the name of the
constraint.
• If you do not know it, you can find the constraint name from
the USER_CONSTRAINTS and USER_CONS_COLUMNS in the
data dictionary.
• The CASCADE option of the DROP clause causes any
dependent constraints also to be dropped.
• Note that when you drop an integrity constraint, that
constraint is no longer enforced by the Oracle Server and is
no longer available in the data dictionary.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
14
Dropping Constraints
• No rows or any data in any of the affected tables are deleted
when you drop a constraint.
ALTER TABLE table_name
DROP CONSTRAINT name [CASCADE}
ALTER TABLE copy_departments
DROP CONSTRAINT c_dept_dept_id_pk CASCADE;
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
15
Disabling Constraints
• By default, whenever an integrity constraint is defined in a
CREATE or ALTER TABLE statement, the constraint is
automatically enabled (enforced) by Oracle unless it is
specifically created in a disabled state using the DISABLE
clause.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
16
Disabling Constraints
• You can disable a constraint without dropping it or re-creating
it by using the ALTER TABLE option DISABLE.
• DISABLE allows incoming data, whether or not it conforms to
the constraint.
• This function allows data to be added to a child table without
having corresponding values in the parent table.
• DISABLE simply switches off the constraint.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
17
Using the DISABLE Clause
• You can use the DISABLE clause in both the ALTER TABLE
statement and the CREATE TABLE statement.
CREATE TABLE copy_employees
( employee_id NUMBER(6,0) PRIMARY KEY DISABLE,
...
...);
ALTER TABLE copy_employees
DISABLE CONSTRAINT c_emp_dept_id_fk;
• Disabling a unique or primary-key constraint removes the
unique index.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
18
Using the CASCADE Clause
• The CASCADE clause disables dependent integrity constraints.
If the constraint is later enabled, the dependent constraints
are not automatically enabled.
• Syntax and example:
ALTER TABLE table_name
DISABLE CONSTRAINT constraint_name [CASCADE];
ALTER TABLE copy_departments
DISABLE CONSTRAINT c_dept_dept_id_pk CASCADE;;
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
19
Enabling Constraints
• To activate an integrity constraint currently disabled, use the
ENABLE clause in the ALTER TABLE statement.
• ENABLE ensures that all incoming data conforms to the
constraint.
• Syntax and example:
ALTER TABLE table_name
ENABLE CONSTRAINT constraint_name;
ALTER TABLE copy_departments
ENABLE CONSTRAINT c_dept_dept_id_pk;
• You can use the ENABLE clause in both the CREATE TABLE
statement and the ALTER TABLE statement.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
20
Enabling Constraint Considerations
• If you enable a constraint, that constraint applies to all the
data in the table.
• All the data in the table must fit the constraint.
• If you enable a UNIQUE KEY or PRIMARY KEY constraint, a
UNIQUE or PRIMARY KEY index is created automatically.
• Enabling a PRIMARY KEY constraint that was disabled with the
CASCADE option does not enable any foreign keys that are
dependent on the primary key.
• ENABLE switches the constraint back on after you switched it
off.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
21
Cascading Constraints
• Cascading referential-integrity constraints allow you to define
the actions the database server takes when a user attempts
to delete or update a key to which existing foreign keys point.
• The CASCADE CONSTRAINTS clause is used along with the
DROP COLUMN clause.
• It drops all referential-integrity constraints that refer to the
primary and unique keys defined on the dropped columns.
• It also drops all multicolumn constraints defined on the
dropped columns.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
22
Cascading Constraints
• If an ALTER TABLE statement does not include the CASCADE
CONSTRAINTS option, any attempt to drop a primary key or
multicolumn constraint will fail.
• Remember, you can't delete a parent value if child values
exist in other tables.
ALTER TABLE table_name
DROP(column name(s)) CASCADE CONSTRAINTS;
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
23
When CASCADE is Not Required
• If all columns referenced by the constraints defined on the
dropped columns are also dropped, then CASCADE
CONSTRAINTS is not required.
• For example, assuming that no other referential constraints
from other tables refer to column PK, it is valid to submit the
following statement without the CASCADE CONSTRAINTS
clause:
ALTER TABLE tablename DROP
(pk_column_name(s));
• However, if any constraint is referenced by columns from
other tables or remaining columns in the target table, you
must specify CASCADE CONSTRAINTS to avoid an error.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
24
Viewing Constraints
• After creating a table, you can confirm its existence by issuing
a DESCRIBE command.
• The only constraint that you can verify using DESCRIBE is the
NOT NULL constraint.
• The NOT NULL constraint will also appear in the data
dictionary as a CHECK constraint.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
25
Viewing Constraints
• To view all constraints on your table, query the
USER_CONSTRAINTS table.
SELECT constraint_name, table_name, constraint_type, status
FROM USER_CONSTRAINTS
WHERE table_name ='COPY_EMPLOYEES';
CONSTRAINT_NAME
TABLE_NAME
CONSTRAINT_TYPE
STATUS
COPY_EMP_PK
COPY_EMPLOYEES
P
ENABLED
CDEPT_DEPT_ID_FK
COPY_EMPLOYEES
R
ENABLED
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
26
Query USER_CONSTRAINTS
• The constraint types listed in the Data Dictionary are:
– P – PRIMARY KEY; R – REFERENCES (foreign key);
– C – CHECK constraint (including NOT NULL);
– U – UNIQUE.
CONSTRAINT_NAME
TABLE_NAME
CONSTRAINT_TYPE
STATUS
COPY_EMP_PK
COPY_EMPLOYEES
P
ENABLED
CDEPT_DEPT_ID_FK
COPY_EMPLOYEES
R
ENABLED
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
27
Terminology
Key terms used in this lesson included:
• ALTER TABLE
• CASCADE clause
• CASCADE CONSTRAINT clause
• DISABLE CONSTRAINT
• DROP COLUMN
• DROP CONSTRAINT
• ENABLE CONSTRAINT
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
28
Summary
In this lesson, you should have learned how to:
• List four different functions that the ALTER statement can
perform on constraints
• Write ALTER TABLE statements to add, drop, disable, and
enable constraints
• Name a business function that would require a DBA to drop,
enable, and/or disable a constraint or use the CASCADE
syntax
• Query the data dictionary for USER_CONSTRAINTS and
interpret the information returned
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
29
14-3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
Objectives
This lesson covers the following objectives:
• List four different functions that the ALTER statement can
perform on constraints
• Write ALTER TABLE statements to add, drop, disable, and
enable constraints
• Name a business function that would require a DBA to drop,
enable, and/or disable a constraint or use the CASCADE
syntax
• Query the data dictionary for USER_CONSTRAINTS and
interpret the information returned
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
3
Purpose
• Would it make any difference if a new student ID number was
entered into the school's database when no actual student
enrolled?
• Is it likely that a credit-card company would issue the same
credit-card number to more than one account or that a
business would hire an employee for a department that
didn't exist?
• What do you predict would happen if a business could not
trust the reliability of the information in its database?
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
4
Purpose
• A database system needs to be able to enforce business rules
and, at the same time, prevent adding, modifying, or deleting
data that might result in a violation of the referential integrity
of the database.
• In this section, you will learn how to make changes to table
constraints so that referential integrity and, in turn, database
reliability are maintained when data needs to be changed.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
5
Managing Constraints
• The ALTER TABLE statement is used to make changes to
constraints in existing tables.
• These changes can include adding or dropping constraints,
enabling or disabling constraints, and adding a NOT NULL
constraint to a column.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
6
Managing Constraints
• The guidelines for making changes to constraints are:
– You can add, drop, enable, or disable a constraint, but you
cannot modify its structure.
– You can add a NOT NULL constraint to an existing column by
using the MODIFY clause of the ALTER TABLE statement.
– MODIFY is used because NOT NULL is a column-level change.
– You can define a NOT NULL constraint only if the table is empty
or if the column contains a value for every row.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
7
The ALTER Statement
• The ALTER statement requires:
– name of the table
– name of the constraint
– type of constraint
– name of the column affected by the constraint
• In the code example shown below, using the employees table,
the primary-key constraint could have been added after the
table was originally created.
ALTER TABLE employees
ADD CONSTRAINT emp_id_pk PRIMARY KEY (employee_id);
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
8
Adding Constraints
• To add a constraint to an existing table, use the following SQL
syntax:
ALTER TABLE table_name
ADD [CONSTRAINT constraint_name] type of constraint (column_name);
• If the constraint is a FOREIGN KEY constraint, the REFERENCES
keyword must be included in the statement.
• Syntax:
ALTER TABLE tablename
ADD CONSTRAINT constraint_name FOREIGN KEY(column_name) REFERENCES
tablename(column_name);
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
9
Adding Constraints Example
• Consider the employees database.
• The primary key from the DEPARTMENTS table is entered in
the EMPLOYEES table as a foreign key.
DEPARTMENTS - Parent
DEPARTMENT_ID
90
110
190
DEPT_NAME
Executive
Accounting
Contracting
MANAGER_ID
100
205
-
LOCATION_ID
1700
1700
1700
FIRST_NAME
Steven
Neena
Lex
Shelley
William
LAST_NAME
King
Kochhar
De Haan
Higgins
Gietz
DEPARTMENT_ID
90
90
90
110
110
EMPLOYEE - Child
EMPLOYEE_ID
100
101
102
205
206
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
10
Adding Constraints Example
• The following example demonstrates the syntax to add this
foreign key to the EMPLOYEES table:
ALTER TABLE employees
ADD CONSTRAINT emp_dept_fk FOREIGN KEY (department_id)
REFERENCES departments (department_id)
ON DELETE CASCADE;
DEPARTMENTS - Parent
DEPARTMENT_ID
90
110
190
DEPT_NAME
Executive
Accounting
Contracting
MANAGER_ID
100
205
-
LOCATION_ID
1700
1700
1700
FIRST_NAME
Steven
Neena
Lex
Shelley
William
LAST_NAME
King
Kochhar
De Haan
Higgins
Gietz
DEPARTMENT_ID
90
90
90
110
110
EMPLOYEE - Child
EMPLOYEE_ID
100
101
102
205
206
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
11
Adding Constraints Conditions
• If the constraint is a NOT NULL constraint, the ALTER TABLE
statement uses MODIFY in place of ADD.
• NOT NULL constraints can be added only if the table is empty
or if the column contains a value for every row:
ALTER TABLE table_name
MODIFY (column_name CONSTRAINT constraint_name NOT NULL);
ALTER TABLE employees
MODIFY (email CONSTRAINT emp_email_nn NOT NULL);
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
12
Why Enable and Disable Constraints?
• To enforce the rules defined by integrity constraints, the
constraints should always be enabled.
• In certain situations, however, it is desirable to temporarily
disable the integrity constraints of a table for performance
reasons, such as:
– When loading large amounts of data into a table
– When performing batch operations that make massive
changes to a table (such as changing everyone's employee
number by adding 1,000 to the existing number)
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
13
Dropping Constraints
• To drop a constraint, you need to know the name of the
constraint.
• If you do not know it, you can find the constraint name from
the USER_CONSTRAINTS and USER_CONS_COLUMNS in the
data dictionary.
• The CASCADE option of the DROP clause causes any
dependent constraints also to be dropped.
• Note that when you drop an integrity constraint, that
constraint is no longer enforced by the Oracle Server and is
no longer available in the data dictionary.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
14
Dropping Constraints
• No rows or any data in any of the affected tables are deleted
when you drop a constraint.
ALTER TABLE table_name
DROP CONSTRAINT name [CASCADE}
ALTER TABLE copy_departments
DROP CONSTRAINT c_dept_dept_id_pk CASCADE;
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
15
Disabling Constraints
• By default, whenever an integrity constraint is defined in a
CREATE or ALTER TABLE statement, the constraint is
automatically enabled (enforced) by Oracle unless it is
specifically created in a disabled state using the DISABLE
clause.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
16
Disabling Constraints
• You can disable a constraint without dropping it or re-creating
it by using the ALTER TABLE option DISABLE.
• DISABLE allows incoming data, whether or not it conforms to
the constraint.
• This function allows data to be added to a child table without
having corresponding values in the parent table.
• DISABLE simply switches off the constraint.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
17
Using the DISABLE Clause
• You can use the DISABLE clause in both the ALTER TABLE
statement and the CREATE TABLE statement.
CREATE TABLE copy_employees
( employee_id NUMBER(6,0) PRIMARY KEY DISABLE,
...
...);
ALTER TABLE copy_employees
DISABLE CONSTRAINT c_emp_dept_id_fk;
• Disabling a unique or primary-key constraint removes the
unique index.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
18
Using the CASCADE Clause
• The CASCADE clause disables dependent integrity constraints.
If the constraint is later enabled, the dependent constraints
are not automatically enabled.
• Syntax and example:
ALTER TABLE table_name
DISABLE CONSTRAINT constraint_name [CASCADE];
ALTER TABLE copy_departments
DISABLE CONSTRAINT c_dept_dept_id_pk CASCADE;;
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
19
Enabling Constraints
• To activate an integrity constraint currently disabled, use the
ENABLE clause in the ALTER TABLE statement.
• ENABLE ensures that all incoming data conforms to the
constraint.
• Syntax and example:
ALTER TABLE table_name
ENABLE CONSTRAINT constraint_name;
ALTER TABLE copy_departments
ENABLE CONSTRAINT c_dept_dept_id_pk;
• You can use the ENABLE clause in both the CREATE TABLE
statement and the ALTER TABLE statement.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
20
Enabling Constraint Considerations
• If you enable a constraint, that constraint applies to all the
data in the table.
• All the data in the table must fit the constraint.
• If you enable a UNIQUE KEY or PRIMARY KEY constraint, a
UNIQUE or PRIMARY KEY index is created automatically.
• Enabling a PRIMARY KEY constraint that was disabled with the
CASCADE option does not enable any foreign keys that are
dependent on the primary key.
• ENABLE switches the constraint back on after you switched it
off.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
21
Cascading Constraints
• Cascading referential-integrity constraints allow you to define
the actions the database server takes when a user attempts
to delete or update a key to which existing foreign keys point.
• The CASCADE CONSTRAINTS clause is used along with the
DROP COLUMN clause.
• It drops all referential-integrity constraints that refer to the
primary and unique keys defined on the dropped columns.
• It also drops all multicolumn constraints defined on the
dropped columns.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
22
Cascading Constraints
• If an ALTER TABLE statement does not include the CASCADE
CONSTRAINTS option, any attempt to drop a primary key or
multicolumn constraint will fail.
• Remember, you can't delete a parent value if child values
exist in other tables.
ALTER TABLE table_name
DROP(column name(s)) CASCADE CONSTRAINTS;
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
23
When CASCADE is Not Required
• If all columns referenced by the constraints defined on the
dropped columns are also dropped, then CASCADE
CONSTRAINTS is not required.
• For example, assuming that no other referential constraints
from other tables refer to column PK, it is valid to submit the
following statement without the CASCADE CONSTRAINTS
clause:
ALTER TABLE tablename DROP
(pk_column_name(s));
• However, if any constraint is referenced by columns from
other tables or remaining columns in the target table, you
must specify CASCADE CONSTRAINTS to avoid an error.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
24
Viewing Constraints
• After creating a table, you can confirm its existence by issuing
a DESCRIBE command.
• The only constraint that you can verify using DESCRIBE is the
NOT NULL constraint.
• The NOT NULL constraint will also appear in the data
dictionary as a CHECK constraint.
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
25
Viewing Constraints
• To view all constraints on your table, query the
USER_CONSTRAINTS table.
SELECT constraint_name, table_name, constraint_type, status
FROM USER_CONSTRAINTS
WHERE table_name ='COPY_EMPLOYEES';
CONSTRAINT_NAME
TABLE_NAME
CONSTRAINT_TYPE
STATUS
COPY_EMP_PK
COPY_EMPLOYEES
P
ENABLED
CDEPT_DEPT_ID_FK
COPY_EMPLOYEES
R
ENABLED
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
26
Query USER_CONSTRAINTS
• The constraint types listed in the Data Dictionary are:
– P – PRIMARY KEY; R – REFERENCES (foreign key);
– C – CHECK constraint (including NOT NULL);
– U – UNIQUE.
CONSTRAINT_NAME
TABLE_NAME
CONSTRAINT_TYPE
STATUS
COPY_EMP_PK
COPY_EMPLOYEES
P
ENABLED
CDEPT_DEPT_ID_FK
COPY_EMPLOYEES
R
ENABLED
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
27
Terminology
Key terms used in this lesson included:
• ALTER TABLE
• CASCADE clause
• CASCADE CONSTRAINT clause
• DISABLE CONSTRAINT
• DROP COLUMN
• DROP CONSTRAINT
• ENABLE CONSTRAINT
DPS14L3
Managing Constraints
Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
28
Summary
In this lesson, you should have learned how to:
• List four different functions that the ALTER statement can
perform on constraints
• Write ALTER TABLE statements to add, drop, disable, and
enable constraints
• Name a business function that would require a DBA to drop,
enable, and/or disable a constraint or use the CASCADE
syntax
• Query the data dictionary for USER_CONSTRAINTS and
interpret the information returned
DPS14L3
Managing Constraints
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